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478 lines
23 KiB
XML
478 lines
23 KiB
XML
<?xml version="1.0"?>
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<html>
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<body>
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<h1 >Firewall and network filtering in libvirt</h1>
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<p>There are three pieces of libvirt functionality which do network
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filtering of some type.
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<br /><br />
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At a high level they are:
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</p>
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<ul>
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<li>The virtual network driver
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<br /><br />
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This provides a isolated bridge device (ie no physical NICs
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enslaved). Guest TAP devices are attached to this bridge.
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Guests can talk to each other and the host, and optionally the
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wider world.
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<br /><br />
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</li>
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<li>The QEMU driver MAC filtering
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<br /><br />
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This provides a generic filtering of MAC addresses to prevent
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the guest spoofing its MAC address. This is mostly obsoleted by
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the next item, so won't be discussed further.
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<br /><br />
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</li>
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<li>The network filter driver
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<br /><br />
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This provides fully configurable, arbitrary network filtering
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of traffic on guest NICs. Generic rulesets are defined at the
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host level to control traffic in some manner. Rules sets are
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then associated with individual NICs of a guest. While not as
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expressive as directly using iptables/ebtables, this can still
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do nearly everything you would want to on a guest NIC filter.
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</li>
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</ul>
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<h3><a name="name-fw-virtual-network-driver"
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id="id-fw-virtual-network-driver">The virtual network driver</a>
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</h3>
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<p>The typical configuration for guests is to use bridging of the
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physical NIC on the host to connect the guest directly to the LAN.
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In RHEL6 there is also the possibility of using macvtap/sr-iov
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and VEPA connectivity. None of this stuff plays nicely with wireless
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NICs, since they will typically silently drop any traffic with a
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MAC address that doesn't match that of the physical NIC.
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</p>
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<p>Thus the virtual network driver in libvirt was invented. This takes
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the form of an isolated bridge device (ie one with no physical NICs
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enslaved). The TAP devices associated with the guest NICs are attached
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to the bridge device. This immediately allows guests on a single host
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to talk to each other and to the host OS (modulo host IPtables rules).
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</p>
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<p>libvirt then uses iptables to control what further connectivity is
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available. There are three configurations possible for a virtual
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network at time of writing:
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</p>
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<ul>
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<li>isolated: all off-node traffic is completely blocked</li>
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<li>nat: outbound traffic to the LAN is allowed, but MASQUERADED</li>
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<li>forward: outbound traffic to the LAN is allowed</li>
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</ul>
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<p>The latter 'forward' case requires the virtual network be on a
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separate sub-net from the main LAN, and that the LAN admin has
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configured routing for this subnet. In the future we intend to
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add support for IP subnetting and/or proxy-arp. This allows for
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the virtual network to use the same subnet as the main LAN and
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should avoid need for the LAN admin to configure special routing.
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</p>
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<p>Libvirt will optionally also provide DHCP services to the virtual
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network using DNSMASQ. In all cases, we need to allow DNS/DHCP
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queries to the host OS. Since we can't predict whether the host
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firewall setup is already allowing this, we insert 4 rules into
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the head of the INPUT chain
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</p>
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<pre>
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target prot opt in out source destination
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ACCEPT udp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 udp dpt:53
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ACCEPT tcp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:53
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ACCEPT udp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 udp dpt:67
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ACCEPT tcp -- virbr0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:67</pre>
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<p>Note we have restricted our rules to just the bridge associated
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with the virtual network, to avoid opening undesirable holes in
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the host firewall wrt the LAN/WAN.
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</p>
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<p>The next rules depend on the type of connectivity allowed, and go
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in the main FORWARD chain:
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</p>
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<ul>
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<li>type=isolated
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<br /><br />
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Allow traffic between guests. Deny inbound. Deny outbound.
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<pre>
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target prot opt in out source destination
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ACCEPT all -- virbr1 virbr1 0.0.0.0/0 0.0.0.0/0
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REJECT all -- * virbr1 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable
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REJECT all -- virbr1 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable</pre>
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</li>
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<li>type=nat
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<br /><br />
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Allow inbound related to an established connection. Allow
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outbound, but only from our expected subnet. Allow traffic
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between guests. Deny all other inbound. Deny all other outbound.
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<pre>
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target prot opt in out source destination
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ACCEPT all -- * virbr0 0.0.0.0/0 192.168.122.0/24 state RELATED,ESTABLISHED
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ACCEPT all -- virbr0 * 192.168.122.0/24 0.0.0.0/0
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ACCEPT all -- virbr0 virbr0 0.0.0.0/0 0.0.0.0/0
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REJECT all -- * virbr0 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable
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REJECT all -- virbr0 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable</pre>
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</li>
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<li>type=routed
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<br /><br />
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Allow inbound, but only to our expected subnet. Allow
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outbound, but only from our expected subnet. Allow traffic
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between guests. Deny all other inbound. Deny all other outbound.
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<pre>
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target prot opt in out source destination
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ACCEPT all -- * virbr2 0.0.0.0/0 192.168.124.0/24
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ACCEPT all -- virbr2 * 192.168.124.0/24 0.0.0.0/0
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ACCEPT all -- virbr2 virbr2 0.0.0.0/0 0.0.0.0/0
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REJECT all -- * virbr2 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable
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REJECT all -- virbr2 * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable</pre>
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</li>
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<li>Finally, with type=nat, there is also an entry in the POSTROUTING
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chain to apply masquerading:
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<pre>
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target prot opt in out source destination
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MASQUERADE all -- * * 192.168.122.0/24 !192.168.122.0/24</pre>
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</li>
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</ul>
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<h3><a name="name-fw-network-filter-driver"
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id="id-fw-network-filter-driver">The network filter driver</a>
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</h3>
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<p>This driver provides a fully configurable network filtering capability
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that leverages ebtables, iptables and ip6tables. This was written by
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the libvirt guys at IBM and although its XML schema is defined by libvirt,
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the conceptual model is closely aligned with the DMTF CIM schema for
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network filtering:
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</p>
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<p><a href="http://www.dmtf.org/standards/cim/cim_schema_v2230/CIM_Network.pdf">http://www.dmtf.org/standards/cim/cim_schema_v2230/CIM_Network.pdf</a></p>
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<p>The filters are managed in libvirt as a top level, standalone object.
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This allows the filters to then be referenced by any libvirt object
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that requires their functionality, instead tieing them only to use
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by guest NICs. In the current implementation, filters can be associated
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with individual guest NICs via the libvirt domain XML format. In the
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future we might allow filters to be associated with the virtual network
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objects. Further we're expecting to define a new 'virtual switch' object
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to remove the complexity of configuring bridge/sriov/vepa networking
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modes. This make also end up making use of network filters.
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</p>
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<p>There are a new set of virsh commands for managing network filters:</p>
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<ul>
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<li>virsh nwfilter-define
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<br /><br />
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define or update a network filter from an XML file
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<br /><br />
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</li>
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<li>virsh nwfilter-undefine
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<br /><br />
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undefine a network filter
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<br /><br />
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</li>
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<li>virsh nwfilter-dumpxml
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<br /><br />
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network filter information in XML
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<br /><br />
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</li>
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<li>virsh nwfilter-list
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<br /><br />
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list network filters
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<br /><br />
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</li>
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<li>virsh nwfilter-edit
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<br /><br />
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edit XML configuration for a network filter
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</li>
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</ul>
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<p>There are equivalently named C APIs for each of these commands.</p>
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<p>As with all objects libvirt manages, network filters are configured
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using an XML format. At a high level the format looks like this:
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</p>
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<pre>
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<filter name='no-spamming' chain='XXXX'>
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<uuid>d217f2d7-5a04-0e01-8b98-ec2743436b74</uuid>
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<rule ...>
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....
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</rule>
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<filterref filter='XXXX'/>
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</filter></pre>
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<p>Every filter has a name and UUID which serve as unique identifiers.
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A filter can have zero-or-more <code><rule></code> elements which
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are used to actually define network controls. Filters can be arranged
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into a DAG, so zero-or-more <code><filterref/></code> elements are
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also allowed. Cycles in the graph are not allowed.
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</p>
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<p>The <code><rule></code> element is where all the interesting stuff
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happens. It has three attributes, an action, a traffic direction and an
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optional priority. eg:
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</p>
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<pre><rule action='drop' direction='out' priority='500'></pre>
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<p>Within the rule there are a wide variety of elements allowed, which
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do protocol specific matching. Supported protocols currently include
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<code>mac</code>, <code>arp</code>, <code>rarp</code>, <code>ip</code>,
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<code>ipv6</code>, <code>tcp/ip</code>, <code>icmp/ip</code>,
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<code>igmp/ip</code>, <code>udp/ip</code>, <code>udplite/ip</code>,
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<code>esp/ip</code>, <code>ah/ip</code>, <code>sctp/ip</code>,
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<code>tcp/ipv6</code>, <code>icmp/ipv6</code>, <code>igmp/ipv6</code>,
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<code>udp/ipv6</code>, <code>udplite/ipv6</code>, <code>esp/ipv6</code>,
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<code>ah/ipv6</code>, <code>sctp/ipv6</code>. Each protocol defines what
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is valid inside the <rule> element. The general pattern though is:
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</p>
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<pre>
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<protocol match='yes|no' attribute1='value1' attribute2='value2'/></pre>
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<p>So, eg a TCP protocol, matching ports 0-1023 would be expressed as:</p>
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<pre><tcp match='yes' srcportstart='0' srcportend='1023'/></pre>
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<p>Attributes can included references to variables defined by the
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object using the rule. So the guest XML format allows each NIC
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to have a MAC address and IP address defined. These are made
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available to filters via the variables <code><b>$IP</b></code> and
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<code><b>$MAC</b></code>.
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</p>
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<p>So to define a filter that prevents IP address spoofing we can
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simply match on source IP address <code>!= $IP</code> like this:
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</p>
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<pre>
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<filter name='no-ip-spoofing' chain='ipv4'>
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<rule action='drop' direction='out'>
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<ip match='no' srcipaddr='<b>$IP</b>' />
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</rule>
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</filter></pre>
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<p>I'm not going to go into details on all the other protocol
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matches you can do, because it'll take far too much space.
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You can read about the options
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<a href="formatnwfilter.html#nwfelemsRulesProto">here</a>.
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</p>
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<p>Out of the box in RHEL6/Fedora rawhide, libvirt ships with a
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set of default useful rules:
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</p>
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<pre>
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# virsh nwfilter-list
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UUID Name
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----------------------------------------------------------------
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15b1ab2b-b1ac-1be2-ed49-2042caba4abb allow-arp
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6c51a466-8d14-6d11-46b0-68b1a883d00f allow-dhcp
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7517ad6c-bd90-37c8-26c9-4eabcb69848d allow-dhcp-server
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3d38b406-7cf0-8335-f5ff-4b9add35f288 allow-incoming-ipv4
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5ff06320-9228-2899-3db0-e32554933415 allow-ipv4
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db0b1767-d62b-269b-ea96-0cc8b451144e clean-traffic
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f88f1932-debf-4aa1-9fbe-f10d3aa4bc95 no-arp-spoofing
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772f112d-52e4-700c-0250-e178a3d91a7a no-ip-multicast
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7ee20370-8106-765d-f7ff-8a60d5aaf30b no-ip-spoofing
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d5d3c490-c2eb-68b1-24fc-3ee362fc8af3 no-mac-broadcast
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fb57c546-76dc-a372-513f-e8179011b48a no-mac-spoofing
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dba10ea7-446d-76de-346f-335bd99c1d05 no-other-l2-traffic
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f5c78134-9da4-0c60-a9f0-fb37bc21ac1f no-other-rarp-traffic
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7637e405-4ccf-42ac-5b41-14f8d03d8cf3 qemu-announce-self
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9aed52e7-f0f3-343e-fe5c-7dcb27b594e5 qemu-announce-self-rarp</pre>
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<p>Most of these are just building blocks. The interesting one here
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is 'clean-traffic'. This pulls together all the building blocks
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into one filter that you can then associate with a guest NIC.
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This stops the most common bad things a guest might try, IP
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spoofing, arp spoofing and MAC spoofing. To look at the rules for
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any of these just do:
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</p>
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<pre>virsh nwfilter-dumpxml FILTERNAME|UUID</pre>
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<p>They are all stored in <code>/etc/libvirt/nwfilter</code>, but don't
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edit the files there directly. Use <code>virsh nwfilter-define</code>
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to update them. This ensures the guests have their iptables/ebtables
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rules recreated.
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</p>
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<p>To associate the clean-trafffic filter with a guest, edit the
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guest XML config and change the <code><interface></code> element
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to include a <code><filterref></code> and also specify the
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whitelisted <code><ip address/></code> the guest is allowed to
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use:
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</p>
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<pre>
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<interface type='bridge'>
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<mac address='52:54:00:56:44:32'/>
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<source bridge='br1'/>
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<ip address='10.33.8.131'/>
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<target dev='vnet0'/>
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<model type='virtio'/>
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<filterref filter='clean-traffic'/>
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</interface></pre>
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<p>If no <code><ip address></code> is included, the network filter
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driver will activate its 'learning mode'. This uses libpcap to snoop on
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network traffic the guest sends and attempts to identify the
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first IP address it uses. It then locks traffic to this address.
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Obviously this isn't entirely secure, but it does offer some
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protection against the guest being trojaned once up and running.
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In the future we intend to enhance the learning mode so that it
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looks for DHCPOFFERS from a trusted DHCP server and only allows
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the offered IP address to be used.
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</p>
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<p>Now, how is all this implemented...?</p>
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<p>The network filter driver uses a combination of ebtables, iptables and
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ip6tables, depending on which protocols are referenced in a filter. The
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out of the box 'clean-traffic' filter rules only require use of
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ebtables. If you want to do matching at tcp/udp/etc protocols (eg to add
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a new filter 'no-email-spamming' to block port 25), then iptables will
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also be used.
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</p>
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<p>The driver attempts to keep its rules separate from those that
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the host admin might already have configured. So the first thing
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it does with ebtables, is to add two hooks in POSTROUTING and
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PREROUTING chains, to redirect traffic to custom chains. These
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hooks match on the TAP device name of the guest NIC, so they
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should not interact badly with any administrator defined rules:
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</p>
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<pre>
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Bridge chain: PREROUTING, entries: 1, policy: ACCEPT
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-i vnet0 -j libvirt-I-vnet0
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Bridge chain: POSTROUTING, entries: 1, policy: ACCEPT
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-o vnet0 -j libvirt-O-vnet0</pre>
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<p>To keep things manageable and easy to follow, the driver will then
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create further sub-chains for each protocol then it needs to match
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against:
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</p>
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<pre>
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Bridge chain: libvirt-I-vnet0, entries: 5, policy: ACCEPT
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-p IPv4 -j I-vnet0-ipv4
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-p ARP -j I-vnet0-arp
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-p 0x8035 -j I-vnet0-rarp
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-p 0x835 -j ACCEPT
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-j DROP
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Bridge chain: libvirt-O-vnet0, entries: 4, policy: ACCEPT
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-p IPv4 -j O-vnet0-ipv4
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-p ARP -j O-vnet0-arp
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-p 0x8035 -j O-vnet0-rarp
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-j DROP</pre>
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<p>Finally, here comes the actual implementation of the filters. This
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example shows the 'clean-traffic' filter implementation.
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I'm not going to explain what this is doing now. :-)
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</p>
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<pre>
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Bridge chain: I-vnet0-ipv4, entries: 2, policy: ACCEPT
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-s ! 52:54:0:56:44:32 -j DROP
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-p IPv4 --ip-src ! 10.33.8.131 -j DROP
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Bridge chain: O-vnet0-ipv4, entries: 1, policy: ACCEPT
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-j ACCEPT
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Bridge chain: I-vnet0-arp, entries: 6, policy: ACCEPT
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-s ! 52:54:0:56:44:32 -j DROP
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-p ARP --arp-mac-src ! 52:54:0:56:44:32 -j DROP
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-p ARP --arp-ip-src ! 10.33.8.131 -j DROP
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-p ARP --arp-op Request -j ACCEPT
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-p ARP --arp-op Reply -j ACCEPT
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-j DROP
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Bridge chain: O-vnet0-arp, entries: 5, policy: ACCEPT
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-p ARP --arp-op Reply --arp-mac-dst ! 52:54:0:56:44:32 -j DROP
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-p ARP --arp-ip-dst ! 10.33.8.131 -j DROP
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-p ARP --arp-op Request -j ACCEPT
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-p ARP --arp-op Reply -j ACCEPT
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-j DROP
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Bridge chain: I-vnet0-rarp, entries: 2, policy: ACCEPT
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-p 0x8035 -s 52:54:0:56:44:32 -d Broadcast --arp-op Request_Reverse --arp-ip-src 0.0.0.0 --arp-ip-dst 0.0.0.0 --arp-mac-src 52:54:0:56:44:32 --arp-mac-dst 52:54:0:56:44:32 -j ACCEPT
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-j DROP
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Bridge chain: O-vnet0-rarp, entries: 2, policy: ACCEPT
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-p 0x8035 -d Broadcast --arp-op Request_Reverse --arp-ip-src 0.0.0.0 --arp-ip-dst 0.0.0.0 --arp-mac-src 52:54:0:56:44:32 --arp-mac-dst 52:54:0:56:44:32 -j ACCEPT
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-j DROP</pre>
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<p>NB, we would have liked to include the prefix 'libvirt-' in all
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of our chain names, but unfortunately the kernel limits names
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to a very short maximum length. So only the first two custom
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chains can include that prefix. The others just include the
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TAP device name + protocol name.
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</p>
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<p>If I define a new filter 'no-spamming' and then add this to the
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'clean-traffic' filter, I can illustrate how iptables usage works:
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</p>
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<pre>
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# cat > /root/spamming.xml <<EOF
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<filter name='no-spamming' chain='root'>
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<uuid>d217f2d7-5a04-0e01-8b98-ec2743436b74</uuid>
|
|
<rule action='drop' direction='out' priority='500'>
|
|
<tcp dstportstart='25' dstportend='25'/>
|
|
</rule>
|
|
</filter>
|
|
EOF
|
|
# virsh nwfilter-define /root/spamming.xml
|
|
# virsh nwfilter-edit clean-traffic</pre>
|
|
|
|
<p>...add <code><filterref filter='no-spamming'/></code></p>
|
|
<p>All active guests immediately have their iptables/ebtables rules
|
|
rebuilt.
|
|
</p>
|
|
<p>The network filter driver deals with iptables in a very similar
|
|
way. First it separates out its rules from those the admin may
|
|
have defined, by adding a couple of hooks into the INPUT/FORWARD
|
|
chains:
|
|
</p>
|
|
<pre>
|
|
Chain INPUT (policy ACCEPT 13M packets, 21G bytes)
|
|
target prot opt in out source destination
|
|
libvirt-host-in all -- * * 0.0.0.0/0 0.0.0.0/0
|
|
|
|
Chain FORWARD (policy ACCEPT 5532K packets, 3010M bytes)
|
|
target prot opt in out source destination
|
|
libvirt-in all -- * * 0.0.0.0/0 0.0.0.0/0
|
|
libvirt-out all -- * * 0.0.0.0/0 0.0.0.0/0
|
|
libvirt-in-post all -- * * 0.0.0.0/0 0.0.0.0/0</pre>
|
|
<p>These custom chains then do matching based on the TAP device
|
|
name, so they won't open holes in the admin defined matches for
|
|
the LAN/WAN (if any).
|
|
</p>
|
|
<pre>
|
|
Chain libvirt-host-in (1 references)
|
|
target prot opt in out source destination
|
|
HI-vnet0 all -- * * 0.0.0.0/0 0.0.0.0/0 [goto] PHYSDEV match --physdev-in vnet0
|
|
|
|
Chain libvirt-in (1 references)
|
|
target prot opt in out source destination
|
|
FI-vnet0 all -- * * 0.0.0.0/0 0.0.0.0/0 [goto] PHYSDEV match --physdev-in vnet0
|
|
|
|
Chain libvirt-in-post (1 references)
|
|
target prot opt in out source destination
|
|
ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 PHYSDEV match --physdev-in vnet0
|
|
|
|
Chain libvirt-out (1 references)
|
|
target prot opt in out source destination
|
|
FO-vnet0 all -- * * 0.0.0.0/0 0.0.0.0/0 [goto] PHYSDEV match --physdev-out vnet0</pre>
|
|
<p>Finally, we can see the interesting bit which is the actual
|
|
implementation of my filter to block port 25 access:
|
|
</p>
|
|
<pre>
|
|
Chain FI-vnet0 (1 references)
|
|
target prot opt in out source destination
|
|
DROP tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:25
|
|
|
|
Chain FO-vnet0 (1 references)
|
|
target prot opt in out source destination
|
|
DROP tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp spt:25
|
|
|
|
Chain HI-vnet0 (1 references)
|
|
target prot opt in out source destination
|
|
DROP tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:25</pre>
|
|
<p>One thing in looking at this you may notice is that if there
|
|
are many guests all using the same filters, we will be duplicating
|
|
the iptables rules over and over for each guest. This is merely a
|
|
limitation of the current rules engine implementation. At the libvirt
|
|
object modelling level you can clearly see we've designed the model
|
|
so filter rules are defined in one place, and indirectly referenced
|
|
by guests. Thus it should be possible to change the implementation in
|
|
the future so we can share the actual iptables/ebtables rules for
|
|
each guest to create a more scalable system. The stuff in current libvirt
|
|
is more or less the very first working implementation we've had of this,
|
|
so there's not been much optimization work done yet.
|
|
</p>
|
|
<p>Also notice that at the XML level we don't expose the fact we
|
|
are using iptables or ebtables at all. The rule definition is done in
|
|
terms of network protocols. Thus if we ever find a need, we could
|
|
plug in an alternative implementation that calls out to a different
|
|
firewall implementation instead of ebtables/iptables (providing that
|
|
implementation was suitably expressive of course)
|
|
</p>
|
|
<p>Finally, in terms of problems we have in deployment. The biggest
|
|
problem is that if the admin does <code>service iptables restart</code>
|
|
all our work gets blown away. We've experimented with using lokkit
|
|
to record our custom rules in a persistent config file, but that
|
|
caused different problem. Admins who were not using lokkit for
|
|
their config found that all their own rules got blown away. So
|
|
we threw away our lokkit code. Instead we document that if you
|
|
run <code>service iptables restart</code>, you need to send SIGHUP to
|
|
libvirt to make it recreate its rules.
|
|
</p>
|
|
<p>More in depth documentation on this is <a href="formatnwfilter.html">here</a>.</p>
|
|
</body>
|
|
</html>
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